Friday, May 22, 2009

The Natural History of Crust

The Earth formed about 4.6 billion years ago and the age of its crust varies greatly across the globe. The basement rock of the Continents is Precambrian rock (igneous and metamorphic), more than a billion years old; indeed, the oldest discovered bedrock is on Greenland, dating back 3.8 billion years. Most of this ancient crust has since been covered by sedimentary and volcanic rocks of more recent eras but it is still visible under three conditions: where it has been uplifted to form mountains, where rivers have cut deep canyons through the overlying sediments or where glaciers have scoured away the more recent deposits. Eventually, even the ancient continental bedrock is recycled, caught in an endless sequence of rifting, collision, uplift and erosion.

Oceanic crust, on the other hand, is far younger. After forming at mid oceanic ridges, this crust moves outward and is eventually recycled at subduction zones, where it dips into oceanic trenches. The oldest oceanic crust on Earth is about 180 million years old, having formed along the East Pacific Rise (in the southeast Pacific) and soon to disappear beneath the North American Plate off eastern Siberia. Since the Continents have been pushed about and bisected by the opening and closing of oceans, the age of the ocean crust, determined by deep sea drilling, provides evidence regarding the history of continental drift. For example, the Atlantic began to open about 160 million years ago, rifting North America from Africa; we know the timing of this event by studying the age of the ocean crust off the opposing coasts of the two continents (it is, indeed, of Jurassic age). Proceeding inward (away from the continents) toward the Mid Atlantic Ridge, the ocean crust is gradually younger and, at the Ridge, new crust is forming today. In the case of Australia and Antarctica, their opposing, offshore oceanic crust is Eocene in age, confirming that their separation, via the Southeast Indian Ridge, began about 50 million years ago.